Did you harden them off before planting them in the ground? It could be that they were in some shade and frequently watered at the store, and now they are just experiencing transplant shock.

What are the soil temps?
Did you water (or rain) since they were transplanted? Even if soil is moist, the rootball may be dry and it takes time to send roots into surrounding soil. Water just the rootball of one plant and see if it perks up. If it does not in a few hours, pull it and slice the stem open to check for brown streaks indicative of verticillium.

Thanks for the responses! Ikea, I tried to pick some eggplants that were in full sun already at the store. Not sure what soil temp is but the air has been around 90 for several days and 80 before that... Slimy Okra, I went out and watered them all around the bases a few minutes ago, and no it has not rained lately. If lack of water is their problem it will be a big relief!! Will post results later (holding breath in the meantime)!

40 minutes after watering the bases... the leaves have popped up like a plastic beach ball that was inflated! Maybe I underestimate how dry the deeper soil is, after 2 weeks of no rain. And it's still around 90, so the "revival" is not because it got cooler! Whew... when I read that midafternoon wilting was a first sign of bacterial wilt, it scared me. I grew one eggplant last year, so 3 plants this year is ambitious for me! Thanks again for suggestions.

Because of the high temperatures, the tops are most likely wilting because their root systems aren't yet well developed.
Rx: Rig temporary shade.

If your plants are not too big you can try putting a laundry basket over them. The idea is to let some sun in (with no heat buildup) in the morning and late afternoon, but at high noon they get maximum protection. My idea assumes you might have a plastic laundry basket with open spaces. This might give the plants a little time to adjust and get some roots gathering moisture better before having to deal with the full on heat. My zone is much different than yours, but I have found the eggplants are in love with water. I water them daily by hand - they respond very well. We are having almost nonstop low to mid 90s. Even so, most of the eggplants are thriving and setting fruit. Since they are thriving so well, I hunted down some more eggplant transplants to try to get in the garden.

I always know mine need water when they get a little droopy. They perk up immediately. In the 100 degree days I need to water the mature eggplants about every three days.